Electrostatic toner polymer particles are commonly prepared by a process commonly referred to as "limited coalescence." In this process, polymer particles having a narrow size distribution are obtained by forming a solution of a polymer in a solvent that is immiscible with water, dispersing the polymer/solvent solution in an aqueous medium containing a solid colloidal stabilizer, and removing the solvent. The resultant particles are then isolated, washed, and dried.
In the practice of this technique, toner particles are prepared from any type of polymer that is soluble in a solvent that is immiscible with water. Thus, the size and size distribution of the resulting particles can be predetermined and controlled by the relative quantities of the particular polymer employed, the solvent, the quantity and size of the water insoluble solid particulate suspension stabilizer and the size to which the solvent-polymer droplets are reduced by the agitation employed.
Limited coalescence techniques of this type have been described in numerous patents pertaining to the preparation of electrostatic toner particles because such techniques typically result in the formation of toner particles having a substantially uniform size and uniform size distribution. Representative limited coalescence processes employed in toner preparation are described in U.S. Pat. Nos. 4,833,060 and 4,965,131.
In the conventional limited coalescence techniques described heretofore, the judicious selection of toner additives such as charge control agents and pigments permits control of the surface roughness of toner particles by taking advantage of the aqueous organic interphase present. However if any toner additive employed for this purpose is highly surface active or hydrophilic in nature, it tends to be present at the surface of the toner particles. Unfortunately, this leads to numerous potential problems such as lower charge/mass, rougher particles, poor thermal transfer, poor electrostatic transfer, reduced pigment coverage, interference with the limited coalescence process, sensitivity to environment, and the like.
Accordingly, workers in the art have recognized that toner additives must either be avoided or kept away from the particle surface. One method for obviating this limitation is to select an additive which is neither surface active or hydrophilic in nature. However, it is often not feasible to find an alternative additive evidencing the properties sought. Thus, it then becomes necessary to modify the surface of the additive.
Efforts to achieve this end have generally focused upon the use of suitable dispersion aids as, for example, polymers with acid or other polar functionality as described in copending application Ser. No. 501,819, filed by M. Nair. Z. Pierce and D. Tyagi. These polymeric stabilizers are used to treat carbon pigment to prevent the hydrophilic carbon from migrating to the surface. Toners prepared in this manner evidence enhanced electrostatic transfer, reduced electrical conductivity, and low D-min or fog.
A similar treatment employed in conjunction with toners of smaller particle size has frequently been found unsuccessful due to an increase in solution viscosity of the organic phase at the concentration of dispersant and pigment level employed. Limited coalescence toners prepared with an organic phase of this type tend to yield particles of broad size distribution and tend to interfere with the limited coalescence process. Furthermore, use of this technique requires the presence of reactive sites on the additive surface of the toner particles to which a dispersant may associate.
Recently, a technique was described in copending application Ser. No. 890,057, filed May 28, 1992, entitled, "Electrostatographic Toner and Method for the Preparation Thereof", for obviating the foregoing limitations wherein a highly surface active pigment was coated with a hydrophobic wax which is insoluble in the solvents commonly employed in toner preparation. Studies revealed that the wax coating prevents the pigment from migrating to the surface of the toner and provides a spherical surface thereto. This results in a narrow distribution of charge from particle to particle and permits image transfer without the occurrence of fog in the background. The wax coating also reduces the likelihood of surface contamination by the pigment due to the fact that it is kept away from the surface. Particulate material prepared in accordance with this technique evidences charging characteristics that render the particles suitable for use as electrostatographic toner particles. Unfortunately, this technique cannot be used when fine dispersion is desired since the melt dispersion process is not efficient. For that purpose, a dispersion made by media milling is appropriate.